Method for producing porous carbon material

ABSTRACT

A novel method for producing a porous carbon material which makes it possible to easily produce a porous carbon material having a desired shape. The method includes immersing a carbon-containing material having a desired shape and composed of a compound, alloy or non-equilibrium alloy containing carbon in a metal bath, the metal bath having a solidification point that is lower than a melting point of the carbon-containing material, the metal bath being controlled to a lower temperature than a minimum value of a liquidus temperature within a compositional fluctuation range extending from the carbon-containing material to carbon by decreasing the other non-carbon main components, to thereby selectively elute the other non-carbon main components into the metal bath while maintaining an external shape of the carbon-containing material to give a porous carbon material having microvoids.

This is a Division of application Ser. No. 16/078,470, which is aNational Phase of International Application No. PCT/JP2016/086357 filedDec. 7, 2016, which claims the benefit of Japanese Application No.2016-041914 filed Mar. 4, 2016. The disclosures of the priorapplications are hereby incorporated by reference herein in theirentireties.

FIELD OF THE INVENTION

The present invention relates to methods for producing porous carbonmaterials and spherical porous carbon materials.

DESCRIPTION OF RELATED ART

Porous carbon materials, such as activated carbon, have beenconventionally applied to a variety of uses including electrodes forvarious kinds of batteries by taking advantage of e.g., a high reactionefficiency due to a huge specific surface area (see, for example, PatentLiterature 1). Since different applications demand different propertiesand qualities in the porous carbon materials, it is a key to obtainporous carbon materials having desired properties and qualities.

Methods for producing porous carbon materials having desired propertiesand qualities include, for example, a method which includes a step ofusing a blend composition to obtain a polymer, the blend compositiongiven by immersing a polymerizable monomer or a composition containingthe monomer in a colloid crystal insoluble to the monomer or to thecomposition, a step of calcination performed under an inert gasatmosphere at 800 to 3000° C., and a step of immersion in a colloidcrystal-soluble solvent to dissolve and remove the colloid crystal, andwhich as a result provides a porous carbon material with voids thatmacroscopically form a crystalline structure and are arranged in aconfiguration having a three-dimensional regularity (see, for example,Patent Literature 2); and a method which includes mixing a polymer Athat is a copolymer of an acrylonitrile monomer and a hydrophilic vinylmonomer, such as a polyacrylonitrile copolymer, with a different kind ofpolymer B in an organic solvent to form an emulsion, bringing theemulsion into contact with a poor solvent for the polymer A toprecipitate the polymer A thereby giving child particle-containingsynthetic resin fine particles, and performing carbonization/calcinationof the child particle-containing synthetic resin fine particles, andwhich as a result provides a porous carbon material having a narrowparticle size distribution and having a porous structure of a specificsize (see, for example, Patent Literature 3).

On the other hand, the present inventors have already developed aso-called dealloying using metallic melt method capable of producingmetal members with micropores on their surface or in their entirety(see, for example, Patent Literature 4).

CITATION LIST

-   Patent Literature 1: JP-B-4762424-   Patent Literature 2: JP-A-2012-101355-   Patent Literature 3: JP-A-2011-225430-   Patent Literature 4: WO 2011/092909

SUMMARY OF THE INVENTION

The methods described in Patent Literatures 2 and 3 for producing porouscarbon materials can produce porous carbon materials having propertiesand qualities that meet respective demands, but a problem with thosemethods is the failure to produce porous carbon materials having adesired shape. Another problem with those methods is complexity of stepsin producing the porous carbon materials.

The present invention has been made focusing on those problems. It is anobject of the present invention to provide a novel method for producinga porous carbon material which makes it possible to easily produce aporous carbon material having a desired shape; and a spherical porouscarbon material.

To attain the above object, the method according to the presentinvention for producing a porous carbon material includes bringing acarbon-containing material having a desired shape and composed of acompound, alloy or non-equilibrium alloy containing carbon into contactwith a molten metal, the molten metal having a solidification point thatis lower than a melting point of the carbon-containing material, themolten metal being controlled to a lower temperature than a minimumvalue of a liquidus temperature within a compositional fluctuation rangeextending from the carbon-containing material to carbon by decreasingthe other non-carbon main components, to thereby selectively elute theother non-carbon main components into the molten metal while maintainingan external shape of the carbon-containing material to give the carbonmaterial having microvoids.

In the method according to the present invention for producing a porouscarbon material, selectively eluting the other non-carbon maincomponents from the carbon-containing material into a molten metal leadsto the repeated bonding between residual carbons, resulting in formingparticles with a nanometer dimension. In addition, this operation causesthose particles to partially bond to one another, consequently givingbulk porous carbon material having microvoids such as meso pores(diameter: 2 nm to 60 nm) and macro pores (diameter: 60 nm or more). Theelution of the other non-carbon main components and the formation andbonding of the particles make progress while maintaining an externalshape of the carbon-containing material. As a result, a porous carbonmaterial having the same shape as the external shape of thecarbon-containing material is obtainable. Thus, the use of acarbon-containing material having a desired shape leads to obtaining aporous carbon material having a desired shape.

The method according to the present invention for producing a porouscarbon material, which utilizes a so-called dealloying using metallicmelt method, is an entirely novel method that has hitherto not existedfor producing a porous carbon material. In the method according to thepresent invention for producing a porous carbon material, the regulationof a temperature of a molten metal suffices in order to obtain theporous carbon material having a desired shape with relative ease at alow cost. In the method according to the present invention for producinga porous carbon material, making changes in a temperature of a moltenmetal, duration of contacting a carbon-containing material with a moltenmetal, and a carbon component proportion within a carbon-containingmaterial, can make a difference in void size and void ratio in a targetporous carbon material.

In the method according to the present invention for producing a porouscarbon material, it is preferred that the carbon-containing material isformed to have a desired shape before being brought into contact withthe molten metal. By doing so, a porous carbon material with any shape,such as a sheet shape and a spherical shape, is readily producible. Inparticular, a possible way is that the carbon-containing material isformed to be spherical by rapidly cooling to solidify acarbon-containing metal melt and thereafter the carbon-containingmaterial with such a shape is brought into contact with the molten metalto thereby give a spherical carbon material having microvoids. In thisway, a spherical porous carbon material is readily producible. Anexemplary way of forming the carbon-containing material that hassphericity is an atomizing method.

In the method according to the present invention for producing a porouscarbon material, as long as it is possible for the other non-carbon maincomponents of the carbon-containing material to be eluted into themolten metal, the carbon-containing material may be brought into contactwith the molten metal in any manner. For instance, a possible way isthat the carbon-containing material is immersed in a metal bath composedof the molten metal to thereby selectively elute the other non-carbonmain components into the metal bath to give the carbon material. Anotherpossible way is that a solid metal having a solidification point that islower than a melting point of the carbon-containing material is arrangedso as to contact the carbon-containing material and thereafter the solidmetal is heated and turned into the molten metal to thereby selectivelyelute the other non-carbon main components into the molten metal to givethe carbon material.

In the method according to the present invention for producing a porouscarbon material, it is preferred that the carbon material is releasedfrom the molten metal and thereafter is subjected to an acid or alkaliaqueous solution to selectively elute and remove an adherent mixturealone that has adhered to a periphery of the carbon material or to aninside of the microvoids and that includes components of the moltenmetal and/or the other non-carbon main components. The use of an acid oralkali aqueous solution enabling the adherent mixture alone to beselectively eluted and not eluting carbon leads to obtaining the porouscarbon material having a desired shape which is composed of carbon as amain component and from which the adherent mixture is removed. Theadherent mixture, before its removal, can adhere to a periphery of theresultant carbon material or partially to an inside of the microvoids,or fill the inside of the microvoids.

In the method according to the present invention for producing a porouscarbon material, it is preferred that the molten metal is composed ofAg, Bi, Cu, Ga, Ge, Hg, In, Ir, Pb, Pt, Rh, Sb, Sn, or Zn, or iscomposed of a mixture that is an alloy of at least one of thosecomponents as a main component, and that the other non-carbon maincomponents are composed of any one or a mixture including more than oneof Al, B, Be, Ca, Ce, Cr, Dy, Er, Eu, Fe, Gd, Hf, Ho, K, La, Li, Lu, Mg,Mn, Mo, Na, Nb, Nd, Pr, Sc, Se, Si, Sm, Sr, Ta, Ti, V, W and Zr. In thiscase, the porous carbon material with a desired shape is producible withparticular efficiency.

In the method according to the present invention for producing a porouscarbon material, it is preferred that the step of selectively elutingthe other non-carbon main components into the molten metal is performedin an inert atmosphere or a vacuum atmosphere, or performed in air withflux added to the molten metal. By doing so, the oxidization of themolten metal can be prevented.

A spherical porous carbon material according to the present inventionhas sphericity and includes microvoids. It is preferred that thespherical porous carbon material according to the present inventioncontains 80% or more of pores ranging in size between 2 to 200 nm in avolume of all pores and has a BET specific surface area of 100 m²/g ormore. It is preferred that the spherical porous carbon materialaccording to the present invention is produced particularly by themethod according to the present invention for producing a porous carbonmaterial.

The present invention provides a novel method for producing a porouscarbon material which makes it possible to easily produce a porouscarbon material having a desired shape; and also provides a sphericalporous carbon material.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 shows a Mn—C phase diagram.

FIG. 2 is a schematic perspective view illustrating (a) a step ofimmersing a carbon-containing material in a metal bath and (b) a step ofwashing a porous carbon material, in a method in an embodiment of thepresent invention for producing a porous carbon material.

FIG. 3 is a schematic perspective view illustrating (a) a step ofremoving an adherent mixture and (b) a step of collecting a porouscarbon material, in a method in an embodiment of the present inventionfor producing a porous carbon material.

FIG. 4 includes (a) a scanning electron microscopic photograph showing acarbon-containing material; (b) a scanning electron microscopicphotograph showing the carbon-containing material at a highermagnification than (a); (c) a scanning electron microscopic photographshowing an obtained porous carbon material; and (d) a scanning electronmicroscopic photograph showing the obtained porous carbon material at ahigher magnification than (c), in a method in an embodiment of thepresent invention for producing a porous carbon material.

FIG. 5 includes (a) a scanning electron microscopic photograph of asheet-shaped porous carbon member; and (b) a scanning electronmicroscopic photograph showing the sheet-shaped porous carbon member ata higher magnification than (a), the sheet-shaped porous carbon memberbeing obtained by a method in an embodiment of the present invention forproducing a porous carbon material.

FIG. 6 illustrates a Raman spectrum of a sheet-shaped porous carbonmember obtained by a method in an embodiment of the present inventionfor producing a porous carbon material.

DETAILED DESCRIPTION OF THE INVENTION

Embodiments of the present invention will be described with reference toexamples hereinafter.

A first thing to do in a method in an embodiment of the presentinvention for producing a porous carbon material is the preparation of aprecursor having a desired shape serving as a carbon-containingmaterial. The precursor is composed of a compound, alloy ornon-equilibrium alloy containing carbon and other non-carbon maincomponents. For instance, reference is made to a Mn—C phase diagramshown in FIG. 1 to prepare a Mn—C precursor alloy where the componentsother than carbon are Mn. In view of the general tendency of Mn and amelt of its alloy being readily oxidizable, the melting is donepreferably in an inert atmosphere such as argon.

Subsequently, as shown in FIG. 2 (a), the prepared precursor, i.e., acarbon-containing material 11, is immersed for a prescribed amount oftime in a metal bath 12 having a solidification point that is lower thana melting point of the carbon-containing material 11. At this time, themetal bath 12 is controlled to a lower temperature than a minimum valueof a liquidus temperature within a compositional fluctuation rangeextending from the carbon-containing material 11 to carbon by decreasingthe other non-carbon main components. For example, in the case of thecarbon-containing material 11 that is a Mn—C precursor alloy, the metalbath 12 is controlled to a lower temperature than a minimum value of aliquidus temperature, 1231° C., in a compositional fluctuation rangeextending to C by decreasing Mn, as determined from the phase diagramshown in FIG. 1. In this case, it is preferred that the metal bath 12 isat 600° C. or higher in view of the fact that reaction is unlikely totake place at a temperature below 600° C.

The duration of the immersion in the metal bath 12 varies depending oncomponents of the metal bath 12 and of the precursor serving as thecarbon-containing material 11, but is around 5 to 10 minutes, forexample, when using the metal bath 12 that is a Bi melt or an Ag meltinto which the carbon-containing material 11 that is a Mn—C precursor isimmersed. When using the metal bath 12 that is a Bi melt into which thecarbon-containing material 11 that is a Mn—C precursor is immersed,floating of the powdery Mn—C precursor occurs on the surface of the meltdue to density difference, so it is preferred during the immersion tostir the precursor and the melt using a rod, for example. In view of thegeneral tendency of Bi and a melt of its alloy being readily oxidizable,performing the dealloying step involving the use of the metal bath 12 isdone preferably in an inert atmosphere such as argon or in a vacuumatmosphere.

The immersion into the metal bath 12 causes the other non-carbon maincomponents (e.g., Mn) to be selectively eluted from thecarbon-containing material 11 into the metal bath 12. This operationleads to the repeated bonding between carbons remaining in the metalbath 12, resulting in forming particles with a nanometer dimension. Inaddition, this operation causes those particles to partially bond to oneanother, consequently giving a bulk porous carbon material 13 havingmicrovoids such as meso pores (diameter: 2 nm to 60 nm) and macro pores(diameter: 60 nm or more). The elution of the other non-carbon maincomponents and the formation and bonding of the particles make progresswhile maintaining an external shape of the carbon-containing material11. As a result, the porous carbon material 13 having the same shape asthe external shape of the carbon-containing material 11 is obtainable.Thus, the use of the carbon-containing material 11 having a desiredshape leads to obtaining the porous carbon material 13 having a desiredshape.

Meanwhile, because of the possibility that an unreacted precursor 14remains at the vicinity of the surface of the melt, a surface of thebulk porous carbon material 13 that has been taken out from the metalbath 12 can have the adhering of the unreacted precursor 14. In viewthereof, as shown in FIG. 2 (b), the unreacted precursor 14 adheringonto the surface of the porous carbon material 13 is removed by washingusing e.g., an ultrasonic cleaning machine.

The periphery of the porous carbon material 13 and the inside of themicrovoids have the adhering of an adherent mixture including componentsof the metal bath 12 and/or the other non-carbon main components (e.g.,Mn). As shown in FIG. 3 (a), in order for the adherent mixture alone tobe selectively eluted and removed, the porous carbon material 13obtained is introduced into an acid or alkali aqueous solution 15. In aninstance shown in FIG. 3 (a), the aqueous solution 15 into which theporous carbon material 13 is introduced is an aqueous nitric acidsolution. First, the porous carbon material 13 is put in e.g., a beaker,into which distilled water in such an amount as will immerse the porouscarbon material is poured. This is followed by a little-by-littleintroduction of nitric acid, which then causes reaction between nitricacid and the adherent mixture, resulting in the elution of the adherentmixture. Thereafter, as shown in FIG. 3 (b), the solid part of theporous carbon material 13 is collected by means such as filtration, andwater-washed and dried. This operation results in giving the porouscarbon material 13 having a desired shape which is composed of carbon asa main component and from which the adherent mixture is removed.

Example 1

By a gas-atomizing method, a precursor serving as a carbon-containingmaterial 11 was produced. Into a coil of a gas-atomizing apparatus(manufactured by Makabe Giken Co., Ltd.), a quartz tube packed with aMnC alloy (Mn:C=85:15 atom %) weighing 60 g was inserted. The inside ofthe gas atomizing apparatus was reduced to around 5×10⁻² Pa. Thereafter,a mixed gas of Ar and H₂ was flowed in to increase the internal pressureto around 111 kPa. By heating at 1400° C., the MnC alloy was molten.After the melting, the molten metal eluted from the nozzle of the quartztube underwent the spraying of an Ar gas at a high pressure (9.5 MPa) tobe crushed to be powdery, and rapidly cooled to be solidified to give aspherical MnC alloy.

Microscopic photographs of the spherical MnC alloy obtained in this wayare shown in FIGS. 4 (a) and (b). As shown in FIGS. 4 (a) and (b), theMnC alloy obtained was found to be spherical and have a particlediameter of not more than 100 μm.

Subsequently, the resultant spherical MnC alloy was used as thecarbon-containing material 11 for the production of a spherical porouscarbon material 13. A metal bath 12 was a Bi melt at 800° C. First, Bihaving a purity of 99.99% (manufactured by Wako Pure ChemicalIndustries, Ltd.) weighing 150 g was introduced into a graphitecrucible. The graphite crucible was inserted into a coil inside ahigh-frequency melting furnace (“VMF-I-I0.5 special-type” manufacturedby DIAVAC LIMITED). The inside of the high-frequency melting furnace wasreduced to around 5×10⁻³ Pa. An argon gas was flowed in to increase thepressure inside the furnace to around 80 kPa, followed by heating.

The heating to 800° C. melted Bi. Thereafter, 3 g of the spherical MnChaving a particle diameter of 20 to 40 μm serving as thecarbon-containing material 11 was introduced into a Bi melt of the metalbath 12. The spherical MnC was held in the metal bath 12 for 10 minutes,and thereafter allowed to cool. The Mn/C/Bi composite after cooled wasintroduced into an aqueous nitric acid solution 15 to dissolve elementsother than C, such as adherent mixtures. Subsequently, the filtrationand pure-water washing were carried out. As a result, the porous carbonmember 13 having C as a main component was obtained.

Microscopic photographs of the porous carbon member 13 obtained in thisway are shown in FIGS. 4 (c) and (d). FIGS. 4 (c) and (d) show theobtainment of the carbon member 13 that was spherical and porous. Theporous carbon member 13 obtained was found to have 91% or more of poresranging in size between 2 to 200 nm in a volume of all pores, and have aBET specific surface area of 128 m²/g.

Example 2

A sheet-shaped porous carbon member 13 was produced. In a firstproduction method, first of all, a precursor MnC thin film (thin film ofMn₈₅C₁₅), serving as a carbon-containing material 11, was formed bysputtering on a Si substrate. The whole of the Si substrate was immersedin a Bi metal bath 12 at 1100° C. for 10 minutes to give thesheet-shaped porous carbon member 13. In order for the periphery of theporous carbon member and the inside of the microvoids to be free of theremaining Mn component and Bi component, the porous carbon member 13 wasimmersed in an aqueous nitric acid solution 15 for 3 hours, which wasfollowed by washing and drying. A scanning electron microscopicphotograph and a Raman spectrum of the sheet-shaped porous carbon member13 obtained in this way are shown respectively in FIG. 5 and in FIG. 6.

In a second production method, a film of Bi was formed on a Sisubstrate, and on the formed film, a precursor MnC thin film (thin filmof Mn₈₅C₁₅), serving as a carbon-containing material 11, was formed bysputtering. The resultant film was subjected to temperature increase to1100° C., and was retained for 10 minutes while promoting a dealloyingreaction between Bi and the precursor. At this time, Bi was molten tobecome a metal bath 12, and into the metal bath 12, Mn was selectivelyeluted from the carbon-containing material 11, resulting in giving thecarbon member 13. After the dealloying step, the whole of the Sisubstrate was cooled, and in order for the periphery of the carbonmember and the inside of the microvoids to be free of the remaining Mncomponent and Bi component, the carbon member 13 was immersed in anaqueous nitric acid solution 15 for 3 hours. This was followed bywashing and drying.

Not just the embodiment of the arrangement of the precursor MnC thinfilm on the Bi thin film, but also embodiments of the arrangement of theprecursor MnC thin film in any manner are permitted as long as the MnCthin film is brought into contact with the Bi that has molten. Forinstance, the MnC thin film may be arranged between the Si substrate andthe Bi thin film, or may be held between the Bi thin films. In the twoproduction methods described above, a thickness of the sheet-shapedporous carbon member 13 is controllable by regulating a thickness of theprecursor MnC thin film and the sputtering time for the formation of theMnC film. A size of the sheet-shaped porous carbon member iscontrollable by regulating a size of the Si substrate and a size of theprecursor MnC thin film.

According to the method in an embodiment of the present invention forproducing a porous carbon material as described above, the regulation ofa temperature of a molten metal suffices in order to obtain the porouscarbon material 13 having a desired shape with relative ease at a lowcost.

In the method in an embodiment of the present invention for producing aporous carbon material, the metal bath 12 is not limited to Bi, but maybe Ag, Cu, Ga, Ge, Hg, In, Ir, Pb, Pt, Rh, Sb, Sn or Zn, or may becomposed of a mixture that is an alloy of at least one of thosecomponents as a main component. The other non-carbon main components ofthe precursor that is the carbon-containing material 11 are not limitedto Mn, but may be composed of any one or a mixture including more thanone of Al, B, Be, Ca, Ce, Cr, Dy, Er, Eu, Fe, Gd, Hf, Ho, K, La, Li, Lu,Mg, Mo, Na, Nb, Nd, Pr, Sc, Se, Si, Sm, Sr, Ta, Ti, V, W and Zr.

For instance, consideration of metal baths (melts) 12 suited for thedealloying step for representative carbon-containing materials(carbides) 11 are thought to provide such results as indicated inTable 1. Table 1 indicates the results of the considerations based onrespective two-dimensional phase diagrams.

TABLE 1 Precursor Melting point (° C.) Melt B₄C 3500 Cu Al₄C₃ 2100 Cu,Zn, Ag, Sn, Pb, Bi SiC 2730 Cu, Ag, CaC₂ 2300 Cu, Zn, Ag, Pb, Bi TiC3170 Cu, Zn, Ag, Sn, Pb, Bi Fe₃C 1250 Sn metastable phase

REFERENCE SIGNS LIST

-   -   11: Carbon-containing material    -   12: Metal bath    -   13: (Porous) carbon member    -   14: Unreacted precursor    -   15: Aqueous solution

What is claimed is:
 1. A method for producing a porous carbon material comprising bringing a starting carbon-containing material having a desired shape and composed of a compound, alloy or non-equilibrium alloy containing carbon into contact with a molten metal to selectively elute non-carbon components into the molten metal while maintaining the external shape of the starting carbon-containing material to give a carbon material having microvoids, wherein the molten metal has a solidification point that is lower than a melting point of the starting carbon-containing material, and a temperature of the molten metal is controlled to be at a temperature that is lower than a minimum value of a liquidus temperature that is observed with respect any composition of the range of compositions that arise as a result of the starting carbon-containing material being converted into the carbon material having microvoids.
 2. The method according to claim 1 for producing a porous carbon material, wherein the starting carbon-containing material is formed to have a desired shape before being brought into contact with the molten metal.
 3. The method according to claim 2 for producing a porous carbon material, wherein the starting carbon-containing material is formed to be spherical by rapidly cooling to solidify a carbon-containing metal melt and thereafter the starting carbon-containing material is brought into contact with the molten metal to thereby give a spherical carbon material having microvoids.
 4. The method according to claim 1 for producing a porous carbon material, wherein the starting carbon-containing material is immersed in a metal bath composed of the molten metal to thereby selectively elute the non-carbon components into the metal bath to give the carbon material.
 5. The method according to claim 1 for producing a porous carbon material, wherein a solid metal having a solidification point that is lower than a melting point of the starting carbon-containing material is arranged so as to contact the starting carbon-containing material, and the solid metal is heated and turned into the molten metal to thereby selectively elute the non-carbon components into the molten metal to give the carbon material.
 6. The method according to claim 1 for producing a porous carbon material, wherein the carbon material is released from the molten metal and thereafter is subjected to an acid or alkali aqueous solution to selectively elute an adherent mixture alone that adheres to a periphery of the carbon material or to an inside of the microvoids and that includes components of the molten metal and/or the non-carbon components.
 7. The method according to claim 1 for producing a porous carbon material, wherein the molten metal is composed of Ag, Bi, Cu, Ga, Ge, Hg, In, Ir, Pb, Pt, Rh, Sb, Sn, or Zn, or is composed of a mixture that is an alloy of at least one of those components as a base metal of the alloy, and wherein the non-carbon components are composed of any one or a mixture including more than one of Al, B, Be, Ca, Ce, Cr, Dy, Er, Eu, Fe, Gd, Hf, Ho, K, La, Li, Lu, Mg, Mn, Mo, Na, Nb, Nd, Pr, Sc, Se, Si, Sm, Sr, Ta, Ti, V, W and Zr.
 8. The method according to claim 1 for producing a porous carbon material, wherein selectively eluting the non-carbon components into the molten metal is performed in an inert atmosphere or a vacuum atmosphere, or performed in air with flux added to the molten metal. 